1. Field of the Invention
The invention relates to a load breaker arrangement for switching a DC current of a DC current circuit on and off in a photovoltaic plant with a semiconductor switching element for preventing a switching arc, there being provided an electronic control unit configured such that one or more signals are received by the control unit and the load breaker arrangement being configured such that in at least one current-carrying line of the DC current circuit there is a galvanic separation by a switching contact that is automatically controllable by the control unit in the switched-off condition and that one or more control signals at the load breaker arrangement are passed to the load breaker arrangement and a semiconductor switch element interrupting the DC current for the switching contact to be de-energized.
Load breaker arrangements for switching a DC current of a DC current circuit on and off in a photovoltaic plant need a suited switching means. The DC current generated by solar energy in photovoltaic modules or in the solar generator, which is converted by an inverter for example into an alternating voltage suitable for a utility grid, can be passed to the inverter or interrupted by the load breaker arrangement.
2. Description of the Prior Art
DC relays are known, which are capable of interrupting a current through switching contacts. The relay incorporates a control coil through which an electric current flows for a metal armature to attract. This metal armature is mechanically connected to one or several electrical main contacts. The main contacts are switched on or off by an attraction movement of the armature toward the control coil depending on whether the relay is configured to be an opening or a closing relay. In principle, such a relay is an electrically controllable switch.
However, relays have the disadvantage that switching arcs occur, in particular when switching off the DC current. This temporarily leads to high temperatures and to a high contact burn of the switching contacts. As a result, the switching contacts can easily fuse together so that they virtually adhere together and are no longer capable of ensuring current interruption. This phenomenon is known under the term of “contact fusing”. Relays are often utilized for lower currents normally not exceeding 30 A.
Comparable electrically controllable switches are termed gates. As compared to relays, they are of a more robust implementation and devised for higher currents such as for up to several 100 A, the risk of contact fusing being reduced. This risk however cannot be completely avoided. The gates however have the disadvantage that they are quite big and also expensive. Moreover, they need a high hold power for the control coil. However, one aims at utilizing the generated energy as effectively as possible not only in photovoltaic plants.
Relay and gate have however the advantage that they can be optionally equipped with additional auxiliary switches connected in series that can be utilized for control purposes. An auxiliary circuit may for example be provided for triggering, through the auxiliary switch, a signal lamp signalizing a failure such as a short circuit.
Other known switching apparatus are load switches or automatic circuit breakers. Such apparatus are fuses or automatically enabling switches that switch off at high current. These apparatus often have an overcurrent and a short circuit function. The overcurrent function is mostly realized by a wire wound about a bi-metal spring and deforming the spring at high current for an enabling mechanism connected to switching contacts to be actuated. This overcurrent protection function is slow. In case of a short circuit or at very high current, very fast electromagnetic forces act very fast upon the enabling mechanism and/or upon the switching contacts themselves so as to allow for fast separation of the switching contacts. In order to reduce occurring switching arcs, quenching plates are utilized as they are also partially used in gates. Accordingly, the risk of contact fusing is quite low. A control of the switching contacts however is only possible with special remote control modules that are very expensive and are only sold for apparatus operating at high current or at high switching powers. If cut off is to occur because of another event such as because a housing cover is opened, solutions with such apparatus are only to be realized for high currents and only at high cost and with considerable space occupancy.
On photovoltaic plants, short circuit currents are normally 20 to 40% higher than in nominal operation because of the characteristic curve typical for solar cells. In such type protection devices however, the current needs to be 50% to 100% higher in order for the short-circuit protection to respond. As a result, a DC current will not be cut off or only late in the event of a short circuit. For safety reasons, this is not acceptable though.
Moreover, the switch-off current is always higher than the nominal current, the fuse cutting off the faster the higher the overcurrent. In case of only low currents, the protection may overheat. Accordingly, such a protection is only conditionally suited for use to protect photovoltaic plants.
Simple fuses are also known. Such a fuse consists of a fuse wire or of a microstrip conductor that is guided and electrically connected with contact elements at the ends of the fuse. The housing can be filled with air, a gas or other fill materials. If the current flowing through the fuse is higher than the nominal current of the fuse, the wire or inserted conductor melts so that the current circuit is interrupted.
Such type fuses are low in cost and very well suited for protecting apparatus and electric components such as circuits or lines and even for very high currents such as several 100 A, and voltages such as several 100 V. A fuse however cannot be reused once it has been enabled and must be replaced. Only an overcurrent comes into consideration as the enabling event. A current interruption due to another event is not possible.
Mechanical DC switches are also known. They comprise a manually operable control element. From outside, this control element is accessible so that it can be brought into one or several other positions through a movement such as a movement of rotation, a pulling or a pushing movement. The manual movement actuates a contact mechanism which in turn actuates an electrical main contact. Such a switch is a manually operable switch with several contact positions that can be achieved by various detent positions of the operating part. They can switch several contacts concurrently in order for example to separate several current circuits at the same time. They are also suited for high switching currents and voltages such as several 100 A and several 100 V. However, they have the disadvantage of requiring much space and of high wiring expense. Moreover, they can only be operated by hand so that they cannot be used for protecting functions such as overcurrent and also not for automatic enablement in other events.
Other current breaker systems suited for photovoltaic plants are known under the term of “Electronic Solar Switch (ESS)” or are described in closer detail in DE 102 25 259 B3 or in DE 10 2004 054 933 B3.
The ESS is disposed in one of the two DC lines. If a mechanical switch is operated, a parallel connected semiconductor switch switches on or off. As a result, arcing is prevented or reduced. Accordingly, the ESS is in principle a manually actuatable switch with electronic assistance for blowing the electric arc out. It is tuned to a photovoltaic inverter and suited for a current of up to more than 10 A and for a voltage of up to more than 100 V. Such an ESS is operable manually only.
The document DE 102 25 259 B3 describes a plug connector connecting a photovoltaic generator to an inverter. Galvanic separation occurs by manually unplugging the plug connector.
In the document DE 10 2004 054 933 B3 there is described a protective cover with electrical contacts and with a handle. By removing the protective cover from an inverter and by subsequently removing the DC plug, a photovoltaic generator is galvanically separated from the inverter.
From the WO 2007/073951 A, a load breaker arrangement for switching off and on a DC current of a DC current circuit in a photovoltaic plant with a semiconductor switch element is known to avoid a switching arc. An electronic control unit is provided for this purpose. In the current carrying lines of the DC current circuit, a galvanic separation occurs through a relay in the switched off condition, namely through two switching contacts that are automatically controllable by control unit. Further, the semiconductor switching element interrupts the DC current so that the switching contacts are cut off. This document describes a switch with a main switching contact and with an auxiliary contact that is connected to the control. According to this solution, a first current path with the main contact and a second current path with a series connection consisting of a semiconductor switch 2 and the relay contact lying on the negative pole side are provided. The second contact, which is mechanically coupled to the relay, lies on the positive pole side.
Another solution with two relays has been described in U.S. Pat. No. 5,633,540. Here, there are provided two relays and one semiconductor switch. A switching contact of the first relay lies in series with the switching element. The switching contact of the other relay lies in parallel with the series connection.